Controlling devices such as these are used in particular as mobile directional control valves for controlling hydraulic consumers, such as, for example, working cylinders and hydraulic motors. Some of these consumers always experience the same direction of force of the external load. Other loads change the direction of their force in operation. The lifting cylinder of a fork lift always experiences a force directed downward, whereas the hydraulic motor of a slewing gear during acceleration can experience a compressive load and, upon braking, a pulling load because the inert mass of the slewing gear continues to run in the original drive direction.
If, at this point, pulling loads move the consumer more quickly than corresponds to the volumetric flow amount in the supply line, the inlet pressure drops rapidly down to the cavitation pressure and below. This occurrence is to be fundamentally prevented.
To counter this, readily available control devices can be purchased on the market which ensure that the cavitation pressure is always reached for the indicated suction action of pulling loads. In the known solutions fluid is supplied by an additional feed system to the respectively endangered pressure line as the supply line. This supply only takes place when the feed pressure applied by this feed system is greater than the pressure in the endangered supply line plus the sum of all pressure drops at the installed throttle points from the supply line to the endangered line. An additional pump system is often encountered as an additional feed system in hydrostatic drives. One option, which is more economical in comparison, arises when the fluid backflow to the tank as a pressure chamber is retained in conventional valve controls by a tank back pressure valve as the control valve. The required supply volume is then taken from this pressure chamber. The disadvantage in these known solutions is the continuing energy loss resulting from the additionally required pump delivery amount and the set back pressure or working capacity of the hydraulic consumer which essentially has been reduced by the back pressure.
To at least partially remedy this, DE 43 42 487 B4 discloses a hydrostatic drive system with a consumer of hydraulic energy supplied on both sides and located in an open circuit. The two ports of the consumer are assigned at least one brake valve with a replenishing valve dynamically connected to it. The replenishing valve enables supply of a hydraulic medium from the outlet side to the inlet side of the consumer. In the known solution, the replenishing valve, in the braking phase in which the brake valve can produce an outlet-side pressure, can be preloaded to an increased replenishing pressure by the pressure produced on the outlet side in the braking phase. In the normal operating state, fluid hydraulic medium can then escape without great resistance by the replenishment valve to the tank. In the braking phase, the replenishment valve is automatically preloaded to a higher opening pressure so that due to the increased replenishment pressure level, external supply to the hydrostatic driving system can be omitted. In this respect, for the known solution the necessity of providing an additional pump system as an auxiliary pump for maintaining a specific inlet-side pressure level is obviated. However, the known solution with a control valve in a double piston execution is complex and therefore expensive to produce.
DE 42 43 578 A1 discloses commercial vehicle hydraulics, in particular for a refuse collection vehicle, with at least one hydraulic circuit. Various actuating elements are connected to that circuit for performing various functions, for example, opening the rear part, lifting and tipping a dumpster, etc. The known solution also has a pump driven by a motor or a secondary output of the commercial vehicle, coupled to it for conveying hydraulic oil into the hydraulic circuit. The pump is designed such that its delivery rate can be controlled at least partially independently of the engine speed. With the known solution, using a control means to determine the power demand of the actuating members connected to the hydraulic circuit, the delivery rate of the pump can always be set such that the engine speed of the commercial vehicle remains as low as possible and is raised only when the power demand is higher. This arrangement helps avoid energy losses.
DE 197 35 482 A1 discloses a hydraulic system with a differential cylinder with a piston rod and piston separating the piston rod-side pressure chamber and the pressure chamber remote from the piston rod from one another. By a directional control valve with two consumer ports, the two pressure chambers of the differential cylinder can be connected alternately to a source of hydraulic medium and to a tank. Independently of this directional control valve, by a quick operating valve, the piston rod-side pressure chamber can be connected to the pressure chamber remote from the piston rod of the differential cylinder. In the known solution, when the quick operating valve is actuated in the rest position or the working position of the directional control valve in which the pressure chamber of the differential cylinder remote from the piston side is supplied with hydraulic medium from the source of a hydraulic medium, collapse of the load is prevented by a check valve located in the connection established by the quick operating valve between the two pressure chambers of the differential cylinder and blocking from the pressure chamber remote from the piston rod to the piston rod-side pressure chamber. The known solution, irrespective of the magnitude of the load being moved with the differential cylinder and counteracting the extension of the piston rod, allows arbitrary actuation of the quick operating valve without endangering anyone and without the risk of damage to the machine so that at any instant a quick traverse motion is possible.